Backbone kwargs in config (#28784)

* Enable instantiating model with pretrained backbone weights

* Clarify pretrained import

* Use load_backbone instead

* Add backbone_kwargs to config

* Pass kwargs to constructors

* Fix up

* Input verification

* Add tests

* Tidy up

* Update tests/utils/test_backbone_utils.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

---------

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

utils/check_config_attributes.py

@@ -224,6 +224,7 @@ def check_attribute_being_used(config_class, attributes, default_value, source_s
         "backbone",
         "backbone_config",
         "use_timm_backbone",
+        "backbone_kwargs",
     ]
     attributes_used_in_generation = ["encoder_no_repeat_ngram_size"]